Electronic labels, such as radio frequency identification (RFID), can be used in non-contact type automatic identification technique. For example, an RFID tag is attached to an object and communicates with an RFID reader through transmission and reception of signals using a radio frequency in order to automatically identify the object using wireless radio waves.
There are different types of electronic labels, with different operating principles and/or functionalities, for example active and passive tags, tag reading only and tag read/write. Read only capability allows the identification of the object, for example, an inventory of objects in a warehouse, or identifications and locations of objects in an automatic manufacturing cycle. Read/write capability can be used in electronic card applications, such as smart card, which needs a read/write feature in order to modify the contents according to the consumption of the user.
The electronic label typically comprises an antenna and an electronic chip. The electronic chip provides the functionality, and the antenna provides the communication with a remote reader. The antenna can also produce the necessary energy to feed the electronic chip through the signal received from the reader. Such tags are intended to be applied to a very large number of objects, for example to shipping boxes, to the individual items in a store, or to credit cards or smart cards.
FIGS. 1A and 1B illustrate an exemplary configuration for communication between an RFID tag and an RFID reader. RFID reader 10 sends signal 11, such as electromagnetic waves, typically at radio frequency, but the waves can be at any desired frequency, either directional or non-directional toward RFID tag 12. The antenna 13 of RFID tag receives the signal 11 and processes the signal with the RFID chip 14. There are various modes of communication, for example, the RFID tag can be an active RFID tag, containing a battery acting as a partial or complete source of power for the tag's circuitry and antenna. The RFID tag can be a passive RFID tag, containing no battery and extracting power from the reader. The RFID tag can receive signals from the RFID reader, and return the signal, usually with some additional data related to the RFID tag. The RFID can actively send back a return signal, or can modulate the signal from the RFID reader so that the RFID reader can receive a modulated scatter return signal. For example, the input impedance of the tag antenna varies in response to the tag identification data, the magnetic field generated from the reader is modulatedly scattered. By receiving the scattered signal, the reader can read the tag identification data, for example, by demodulating the receiving scattered signal.
There is a requirement for the reading of an RFID tag: the tag antenna has to receive the signal sent from the RFID reader. For example, if the tag is too far, e.g., out of the range of the reader, then the reader cannot “see” the tag. Alternatively, if the tag is located in a dead spot, e.g., a location that the signal cannot reach, the tag is not responsive to the inquiry from the reader.
A potential dead spot for the magnetic field is the proximity of a conductive object, such as at a metal surface. When a magnetic flux encounters metals, eddy currents flow on the metal surface, generating a magnetic field that opposes the coming magnetic flux. The net effect is a shielding effect where the magnetic flux from the reader avoids the metallic surface, effectively staying away from the tag antenna.
FIG. 2A illustrates a tag 12 disposed on a surface of a non-metal material 25. The magnetic field 11 can pass through the non-metal material 25, thus allowing the tag 12 to receive the signal from a reader. FIG. 2B illustrates a tag 12 disposed on a surface of a metal 26. The magnetic field 23 is bent around the metal 26, effectively preventing the tag 12 from receiving the signal. An explanation for the repelling of magnetic field near the metal surface can be as follows. Without the metal 26, the magnetic field lines can be as shown in 21. The magnetic field lines 21 pass through the metal, generating an eddy current on the metal surface. The eddy current then generates an opposing magnetic field 22. The combination of the original magnetic field 21 and the generated magnetic field 22 becomes the final magnetic field 23, which is repelled from the metal surface.
To prevent this loss of signals, the reader can be brought nearer the tag to restore the communication. Alternatively, the tag can be positioned farther from the metal surface, for example, by an insulating support layer. However, these solutions are often not practical.
A potential prior art solution is the introduction of a high permeable material on the metal surface. This high permeable layer can reduce the eddy current to enable tag placement on a metal surface through the high permeable layer. FIGS. 3A and 3B illustrate the effect of this high permeable layer. In FIG. 3A, a tag 12 is placed on a metal surface 25. The magnetic field 23 is repelled from the metal 25, reducing or preventing the tag 12 from receiving the signal. In FIG. 3B, a high permeable layer 32 is placed on the metal surface 25 and the tag 12 is disposed on the high permeable layer 32. Since magnetic field can pass more easily through highly permeable material than through low permeable material, the magnetic field is modified, resulting in a magnetic field 33 that passes through the highly permeable layer 32. With the tag 12 disposed on the high permeable layer 32 and intercepting the magnetic field 33, the tag can now receive the signal. The highly permeable layer sometimes can be called magnetic absorbing material, since it absorbs magnetic field. In some literature, the highly permeable layer is called magnetic reflective material, since the magnetic field is reflected from the surface of substrate 25.